Catalytic converters are used in motor vehicles for exhaust gas treatment to reduce harmful emissions in the exhaust gas. There are various known types of catalytic converters, such as three-way catalytic converters, unregulated oxidation catalytic converters and SCR catalytic converters.
In SCR catalytic converters, so-called selective catalytic reduction (SCR: Selective Catalytic Reduction) is used as a method for the reduction of nitrogen oxides. The chemical reaction in an SCR catalytic converter is selective, thus the nitrogen oxide (NO, NO2) is reduced, while undesired side reactions, such as oxidation of the sulfur to sulfur dioxide, are largely suppressed.
In internal combustion engines used in motor vehicles, the reduction of nitrogen oxides by the SCR method proves to be difficult because there exist varying operating conditions, which makes the dosage of reducing agents difficult.
A reducing agent is dosed for the operation of SCR catalytic converters, whereby a NOx sensor value is controlled after SCR. The NOx sensor has a cross-sensitivity to NH3. If overdosing takes place in the system, then a so-called NH3 slip is the result after the SCR reaction, or increased NOx emissions arise again after the SCR if a slip catalyst is used in the system after the SCR.
The NO sensor therefore presents ambiguity in a characteristic curve. Therefore, it cannot be predictably differentiated whether the dosage is too low and whether NOx emissions are present, or whether the dosage is too high and a NH3 slip or increased NOx emission is present due to NH3 conversion in the slip catalyst.
The problem described does not occur when only low NOx conversion rates are required through the SCR catalytic converter. Then the conversion in the system is far from the maximum possible conversion with the so-called slip limit. However, high conversion rates must be achieved for fuel-saving engine tuning and efficient utilization of the catalytic converter.
Another way to resolve the ambiguity of the characteristic curve is to introduce artificially small changes in the dosage quantity of the reducing agent. By appropriate evaluation of the NOx value after the SCR, the presence of an NH3 slip can be detected. Such an approach is described in DE 10 2009 012 092 A1.
Such an evaluation, however, only works when the system is in a steady state; thus the existence of an NH3 slip is only detected after a certain delay.